Condensed Matter > Strongly Correlated Electrons

Title:Theory of Spin Exciton in the Kondo Semiconductor $Yb B_{12}$

Abstract: The Kondo semiconductor $Yb B_{12}$ exhibits a spin and charge gap of
approximately 15 meV. Close to the gap energy narrow dispersive collective
excitations were identified by previous inelastic neutron scattering
experiments. We present a theoretical analysis of these excitations. Starting
from a periodic Anderson model for crystalline electric field (CEF) split 4f
states we derive the hybridized quasiparticle bands in slave boson mean-field
approximation and calculate the momentum dependent dynamical susceptibility in
random phase approximation (RPA). We show that a small difference in the
hybridization of the two CEF (quasi-) quartets leads to the appearance of two
dispersive spin resonance excitations at the continuum threshold. Their
intensity is largest at the antiferromagnetic (AF) zone boundary point and they
have an upward dispersion which merges with the continuum less than halfway
into the Brillouin zone. Our theoretical analysis explains the most salient
features of previously unexplained experiments on the magnetic excitations of
$Yb B_{12}$.